Gas Anti-Solvent (GAS) Recrystallization is a recently developed process for recrystallizing materials that are ordinarily difficult-to-comminute. Supercritical fluids (SCF) and additionally gases at conditions near their respective vapor pressures (hence the term GAS) have the ability to dissolve in and expand liquid solutions. GAS Recrystallization exploits this property by using gases or SCFs as anti-solvents for inducing nucleation of a dissolved solid from an organic solution.
RDX (cyclotrimethylenetrinitramine) and HMX (cyclotetramethylenetetranitramine) are energetic materials which are used in explosive and propellant formulations. It is difficult to process these materials by current methods not only because of their sensitivity but also because conventional techniques for recrystallization do not always form crystals of desirable size, size distribution, shape, or morphology. Previous research has shown that GAS Recrystallization can successfully recrystallize RDX, forming particles of desirable characteristics. HMX is a byproduct of RDX production and its presence can significantly effect further RDX processing, and therefore, it would be advantageous to be able to separate these two materials. The GAS process can be tailored so as to selectively precipitate the RDX thereby forming an essentially HMX-free product. Subsequent processing of the spent solution may allow one to recover the HMX, itself a high value product.
Current manufacture of RDX and HMX utilizes processes that were developed decades ago. The Bachmann process for RDX production.sup.1 results in the formation of a by-product, HMX, which is present in the product from 2 to 20 wt %. The structures of RDX and HMX are: ##STR1##
Besides by-product formation, a major drawback of the conventional manufacturing techniques for RDX is that the product is generally not of a desirable crystal size, size distribution, shape, or morphology, and further recrystallization is necessary; pure forms of both RDX and HMX can be recrystallized by thermally induced precipitation from a solvent, but the presence of impurities can have a significant effect on the resultant crystals.
While RDX shows no evidence of room temperature stable polymorphism it can exist in many different crystal habits. HMX, on the other hand, exists in four polymorphs (.alpha., .beta., .gamma., .delta.), each having different stability ranges and physical properties. In order to obtain one form exclusive of the others by current recrystallization techniques one must precisely control the temperature, the cooling rate, and the degree of agitation. To complicate matters further, the HMX polymorphs can undergo transformations which can be influenced not only by the "environmental" conditions during recrystallization but also by the presence of RDX. It is speculated that, likewise, the presence of HMX can influence RDX recrystallization and may possibly contribute to the formation of voids during RDX crystal growth. FIG. 1 shows a typical sample of as-produced RDX; the dark spots are large, intragranular cavities made visible using refractive index matching fluid. (In an aside, HMX-free RDX has been produced in laboratory quantity.sup.2, but the recrystallized material does contain intragranular cavities.) These flaws in crystal struture can adversely affect the performance of the overall explosive or propellant formulations. However, recent developments have indicated that while small amounts of HMX in the RDX can be undesirable, high HMX-content RDX (greater than 30 wt % HMX) may be a potentially new, highly energetic combined product explosive (CPX).sup.3.
Supercritical fluids have been investigated as solvents for extraction, purification, and recrystallization based on their pressure-dependent dissolving properties. For the past few years Phasex has been developing a process, called Gas Anti-Solvent (GAS) Recrystallization, for processing "sensitive" materials such as pharmaceuticals and explosives. Application of the process to the recrystallization of nitroguanidine (NQ).sup.4, RDX.sup.5, and other materials.sup.6 has been described elsewhere but a brief review is given here in order to clarify further discussion. Since they are generally miscible with many organic liquids, supercritical fluids and even gases can be effective anti-solvents for recrystallization of materials that are insoluble in "simple" supercritical fluids. Introduction of the gas into an organic solvent results in an expansion of the liquid phase as more of the gas is dissolved into the liquid. The expansion curves for neat acetone/CO.sub.2 system shown in FIG. 2 depict the miscibility characteristics between the solvent and anti-solvent; although the absolute numbers are different, the same general trend arises with many solvent/gas anti-solvent pairs. If a solution of organic solvent plus dissolved solid is expanded in a similar manner, supersaturation in the solution is created. At some point, a "critical" supersaturation level is exceeded and the solid precipitates. By varying the operating conditions including temperature, pressure, and initial solution concentration, as well as other variables such as the solvent itself and the rate of anti-solvent addition, one can produce a variety of crystal sizes, size distributions, and habits. FIGS. 3, 4, and 5 show select samples that were produced on an initial Ballistic Research Laboratory-funded program (cf. ref. 5); nearly all the crystals formed were essentially void-free as determined by optical microscopy using refractive index matching fluid.
Recent interest has focused on using GAS Recrystallization not only for producing void-free RDX but also for separating RDX and HMX. The GAS process has the potential for achieving this separation based on a trend exhibited by all of the various systems tested at Phasex: more dilute solutions require higher levels of expansion (and correspondingly higher pressures) before the "onset of nucleation" occurs. FIG. 6 depicts this general relation between initial solution concentration and expansion required for nucleation; the pressure at which the onset of visible nucleation occurs has been termed threshold pressure (THP), and predetermined expansion curves (for example, FIG. 2) permit direct correlation between expansion levels and pressure.
Therefore, if a solvent which dissolves both RDX and HMX to about the same weight percent (assuming the maximum of 20% HMX in the RDX and that the solution is significantly less saturated in HMX than RDX) is chosen, it is possible to recover RDX in a "first" expansion, then to recover HMX (or at least a CPX) in a "second" higher level expansion. One might ask, "Why not just choose a solvent which can dissolve the RDX and not HMX and then just filter out the HMX?". RDX and HMX are very close in chemical structure and the two tend to strongly "associate" with one another making separation by this method nearly impossible.
Evaluating suitable solvents is more complex than simply looking at their ability to dissolve the solid RDX/HMX mixture; the crystal habit and morphology of the recrystallized material must be of a desirable, void-free form. There should be no chemical reaction or complex formation as is the case with HMX and dimethyl formamide, for example. Also, at least partial miscibility between the solvent and gas anti-solvent must exist. For this study, several solvents were evaluated but for brevity, only the most promising will be discussed further.